Optical communication systems which operate in the wavelength range between about 1100 and 1700 nanometers (nm) are of potentially great importance because the dispersion and losses in an optical fiber are typically very low in this wavelength range. Heterojunction devices incorporating binary III-V alloys and solid solutions of these alloys have been found to be particularly useful for this application because their electronic bandgaps occur in this wavelength range and lattice-matched heterojunctions can be obtained by compositional variation. In particular ternary and quaternary alloys of InGaAsP on an InP substrate have been found to be useful materials for both light emitters and detectors. It should be understood, however, that the quaternary alloys usually prove move difficult to grow due to the balancing of the four elements to provide lattice-matched heterojunctions.
Problems which affect the performance of avalanche photodetectors using these materials include bulk tunneling currents which occur at electric fields above about 1.5.times.10.sup.5 V/cm in the ternary and quaternary compound used for the light absorptive region, edge breakdown, and multiplication of surface leakage currents at the junction periphery. The tunneling has been reduced by locating the PN junction with its high electric field in a wide bandgap material separated from the light absorptive region in the narrower bandgap material.
Edge breakdown and surface leakage currents have been reduced by the use of surface contouring of the detector sidewalls. However, the electric field reduction at the surface may be small with the result that the surface leakage current may still undergo multiplication. To reduce the multiplication of surface leakage current and to enhance multiplication in an active region away from the heterojunction formed between the active region and a light absorptive region, I provide in my U.S. Pat. No. 4,700,209 issued Oct. 13, 1987, a photodetector having a light absorptive region, an active region overlying the light absorptive region and a cap region of opposite conductivity type to the other two regions overlying the active region. A silicon implanted central zone is located in the active region of an avalanche photodetector. The central zone has a greater concentration of first type conductivity modifiers than the remainder of the active region. My aforementioned U.S. Pat. No. 4,700,209 specifically teaches that the central zone is located spatially separated from the cap and absorptive regions within the active region. The use of the silicon implanted central zone in this photodetector has resulted in achieving multiplication in the active region with electric fields in the order of 4 to 5.times.10.sup.5 V/cm while at the same time maintaining the electric field at the heterojunction below the implanted central zone to between 1.5 to 2.times.10.sup.5 V/cm so that no significant tunneling in the absorptive region occurs.
My aforementioned U.S. Pat. No. 4,700,209 further discloses an intermediate quaternary layer between the active region and the absorptive region to avoid slow detector response times. It should be understood that a quaternary layer of InGaAsP is more difficult to grow than the remaining layers since the amounts of the four elements must be balanced in specific proportions to obtain a lattice matched heterojunction with the active region, usually InP, and the absorptive region, usually InGaAs.
It would be desirable to maintain response times compatible with those achieved by using an intermediate quaternary layer between the active region and light absorptive layer but without necessarily having to use the quaternary layer.